Change display resolution in Redhat 5 or Oracle Enterprise Linux 5

Edit your "/etc/X11/xorg.conf" file, add the "Modes" part under subsection "Display" where the first mode - which is 1024x768 in the example below - will be the default one:

Section "Screen"
        Identifier "Screen0"
        Device     "Videocard0"
        DefaultDepth     24
        SubSection "Display"
                Viewport   0 0
                Depth     24
                Modes     "1024x768" "800x600" "640x480"
        EndSubSection
EndSection


Save the file, then restart your X server or restart your computer if you don't know how to :)

Source